This invention relates generally to electrostatic discharge clamps, and more particularly to a voltage clamp employing a vertical NPN transistor structure.
The input terminals to bipolar circuits must be provided with electrostatic discharge clamps in order to protect the bipolar circuitry, especially sensitive input structures. The human body, for example, can be modeled as a charged 100 PF capacitor in series with a 1.5 K ohm resistor with capacitor voltages sometimes exceeding 10 KV. Thus, a human body can provide peak voltage and current sufficient to damage circuits designed to operate at a low voltage level.
Protection against electrostatic discharge voltages is conventionally provided by placing or connecting input terminals to a negative doped region, in a semiconductor substrate which has a positive dopant concentration. The resulting PN junction, or diode, will clamp the input terminal to a low negative voltage relative to the substrate value by discharging a large negative voltage to the substrate. This is much less effective for positive electrostatic discharge voltages, however, because the reverse biased diode is less capable of clamping at a voltage low enough to prevent damage. To be useful, electrostatic discharge circuits must clamp well in both directions.
Protection against voltage discharge has heretofore been provided in MOS circuits by connecting the input terminal to a lateral NPN transistor. This invention is directed to a novel clamp particularly useful with bipolar circuits and biCMOS circuits and which can be effective for positive and negative electrostatic discharge voltages.